This invention is generally directed to developer compositions, and more specifically, the present invention relates to developer compositions with coated carrier particles prepared by a solution process. In one embodiment of the present invention, the carrier particles are comprised of a core with coating thereover generated from a solution mixture of polymers that are not in close proximity thereto in the triboelectric series. In another aspect of the present invention, the carrier particles are prepared by a solution coating process wherein a mixture of two polymers, at least one, and preferably one, of which is substantially soluble in the solvent selected, are applied to the carrier core enabling insulating particles with relatively constant conductivity parameters; and also wherein the triboelectric charge, either positive or negative on the carrier can vary significantly depending on the coatings selected. Developer compositions comprised of the carrier particles prepared by the solution coating process of the present invention are useful in electrostatographic or electrophotographic imaging systems, especially xerographic imaging processes. Additionally, developer compositions comprised of substantially insulating carrier particles prepared in accordance with the process of the present invention are useful in imaging methods wherein relatively constant conductivity parameters are desired. Furthermore, in the aforementioned imaging processes the triboelectric charge, and/or the conductivity of the carrier particles can be preselected depending on the polymer composition applied to the carrier core.
The electrostatographic process, and particularly the xerographic process, is well known. This process involves the formation of an electrostatic latent image on a photoreceptor, followed by development, and subsequent transfer of the image to a suitable substrate. Numerous different types of xerographic imaging processes are known wherein, for example, insulative developer particles or conductive toner compositions are selected depending on the development systems used. Moreover, of importance with respect to the aforementioned developer compositions is the appropriate triboelectric charging values associated therewith as it is these values that can enable continued constant developed images of high quality and excellent resolution.
In a patentability search report, the following United States Patents were recited: U.S. Pat. No. 4,331,756 which discloses that a carrier core can be coated with a solution containing a mixture of butadiene/acrylonitrile rubber and polyurethane, and also see column 8, lines 1 to 7, and column 5, lines 14 to 30; U.S. Pat. Nos. 3,778,262; 3,873,355 and 3,873,356 wherein there is indicated that the very negative tribo that would result from a carrier coated with PTFE is raised to a more positive tribo by mixing another type of resin with the PTFE and solution coating the mixed resins on the carrier core, and note as indicated in the Abstract of the Disclosure that the carrier core is coated with a continuous film of a fluoropolymer and a modifying resin, see the U.S. Pat. No. 3,778,262 patent and also note Examples 1 and 2 in columns 5 and 6; and U.S. Pat. Nos. 4,937,166 and 4,935,326 wherein there are illustrated carrier compositions comprised of a core with a coating thereover comprised of a mixture of a first and second polymer that are not in close proximity thereto in the triboelectric series, which coatings are applied by a dry coating process, reference the '166 patent, and a developer containing a toner with a suspension polymerized styrene butadiene and other components in a carrier with a coating, which carrier is similar to the carrier as illustrated in the '166 patent. The disclosures of each of the aforementioned patents are totally incorporated herein by reference.
Carrier particles for use in the development of electrostatic latent images are described in many patents including, for example, U.S. Pat. No. 3,590,000. These carrier particles may be comprised of various cores, including steel, with a coating thereover of fluoropolymers, and terpolymers of styrene, methacrylate, and silane compounds. Efforts have focused on the attainment of coatings for carrier particles for the purpose of improving development quality, and also to permit particles that can be recycled, and that do not adversely effect the imaging member in any substantial manner. A number of the present commercial carrier coatings can deteriorate rapidly, especially when selected for a continuous xerographic process where the entire coating may separate from the carrier core in the form of chips or flakes, and fail upon impact, or abrasive contact with machine parts and other carrier particles. These flakes or chips, which cannot generally be reclaimed from the developer mixture, have an adverse effect on the triboelectric charging characteristics of the carrier particles thereby providing images with lower resolution in comparison to those compositions wherein the carrier coatings are retained on the surface of the core substrate. Further, another problem encountered with some prior art carrier coatings resides in fluctuating triboelectric charging characteristics, particularly with changes in relative humidity. The aforementioned modification in triboelectric charging characteristics provides developed images of lower quality, and with background deposits.
There are also illustrated in U.S. Pat. No. 4,233,387, the disclosure of which is totally incorporated herein by reference, coated carrier components for electrostatographic developer mixtures comprised of finely divided toner particles clinging to the surface of the carrier particles. Specifically, there are disclosed in this patent coated carrier particles obtained by mixing carrier core particles of an average diameter of from between about 30 microns to about 1,000 microns with from about 0.05 percent to about 3.0 percent by weight, based on the weight of the coated carrier particles, of thermoplastic resin particles. The resulting mixture is then dry blended until the thermoplastic resin particles adhere to the carrier core by mechanical impaction, and/or electrostatic attraction. Thereafter, the mixture is heated to a temperature of from about 320.degree. F. to about 650.degree. F. for a period of 20 minutes to about 120 minutes enabling the thermoplastic resin particles to melt and fuse on the carrier core. While the developer and carrier particles prepared in accordance with the process of this patent, the disclosure of which has been totally incorporated herein by reference, are suitable for their intended purposes, the conductivity values of the resulting particles are not constant in all instances, for example, when a change in carrier coating weight is accomplished to achieve a modification of the triboelectric charging characteristics; and further with regard to the'387 patent, in many situations carrier and developer mixtures with only specific triboelectric charging values can be generated when certain conductivity values or characteristics are contemplated. With the invention of the present application, the conductivity of the resulting carrier particles can be substantially constant, and moreover the triboelectric values can be selected to vary significantly, for example, from less than -15 microcoulombs per gram to greater than -70 microcoulombs per gram, or from less than a positive 15 microcoulombs per gram to greater than a positive 70 microcoulombs per gram depending, for example, on the carrier core and polymer mixture selected for affecting the coating process.
There are illustrated in U.S. Pat. No. 4,937,166 and 4,935,326, the disclosures of which are totally incorporated herein by reference, carrier particles with, for example, a mixture of coatings, such as two coatings not in close proximity in the triboelectric series to enable control of the conductivity thereof independent of the triboelectric charging values for example. The aforementioned carriers according to the aforementioned patents are prepared by dry coating processes. Advantages associated with the solution coated carriers of the present invention as compared to the dry coated carriers of the '166 and 326 patents include independence from particle size constraint, that is for example the polymers selected for the aforementioned dry coating can be of a small particle size, for example about 1 micron in average diameter, to enable the polymers to effectively fuse to the core, a greater variety of polymers are available for solution coatings enabling a more complete control of the carrier characteristics, and the like.
It is known that carriers obtained by applying a single insulating resinous coating to porous metallic carrier cores using solution coating techniques can be undesirable from many viewpoints. For example, the coating material may reside in the pores of the carrier cores, rather than at the surfaces thereof; and therefore is not available for triboelectric charging when the coated carrier particles are mixed with finely divided toner particles. Attempts to resolve this problem by increasing the carrier coating weights, for example, to as much as 3 percent or greater to provide an effective triboelectric coating to the carrier particles necessarily involves handling excessive quantities of solvents, and further these processes can result in low product yields. When resin coated carrier particles are prepared by powder coating processes, the majority of the coating materials are fused to the carrier surface thereby reducing the number of toner impaction sites on the carrier material. Additionally, there can be achieved with the dry coating process independent of one another desirable triboelectric charging characteristics and conductivity values; that is, for example, the triboelectric charging parameter is not dependent on the carrier coating weight as is believed to be the situation with the process of U.S. Pat. No. 4,233,387 wherein an increase in coating weight on the carrier particles may function to also permit an increase in the triboelectric charging characteristics.
Other patents that may be of interest include U.S. Pat. No. 3,939,086, which teaches steel carrier beads with polyethylene coatings, see column 6; U.S. Pat. No. 4,264,697, which discloses dry coating and fusing processes; U.S. Pat. Nos. 3,533,835; 3,658,500; 3,798,167; 3,918,968; 3,922,382; 4,238,558; 4,310,611; 4,397,935; and 4,434,220.
There can be formulated in accordance with the invention of the present application developers with conductivities of from about 10.sup.-6 mho (cm).sup.-1 to 10.sup.-17 mho (cm).sup.-1 as determined in a magnetic brush conducting cell, and triboelectric charging values of from about a positive or negative 8 to 80 microcoulombs per gram on the carrier particles as determined by the known Faraday Cage technique. Thus, the developers of the present invention can be formulated with constant conductivity values with different triboelectric charging characteristics by, for example, maintaining the same coating weight on the carrier particles and changing the polymer coating ratios. Similarly, there can be formulated developer compositions wherein constant triboelectric charging values are achieved and the conductivities are altered by retaining the polymer ratio coating constant and modifying the coating weight for the carrier particles.